This invention relates to a coordinate reading device utilized as an input device for computers.
FIG. 3 shows the construction of a prior art coordinate reading device. A sense-line device 1 is scanned succesively through X and Y axis scanning circuits 4a and 4b by a scanning address signal 3 sent from the control circuit 2. The signal generated at a coordinate indicator 6 by a drive circuit 5 is input into the control circuit 2 after the signal is detected by the sense-line device 1, then amplified and waveformed by an analogue circuit 5, and converted into a digital signal 9 by an A/D converter 8. At this point, it is known that the control circuit 2 determines the position of the coordinate indicator 6 according to the scanning address signal 3 and the signal 9 fed from the A/D converter. For example, this kind of prior art device for reading coordinates is disclosed by the applicant in the Japanese Patent publication No. 55-99683.
In the prior art coordinate reading device, the sense-line device 1 is constructed by electrodes, for example, a print-base-board or an insulating board 1c, and the sense-line 1a in the X axis direction and the sense-line electrodes 1b in the Y axis direction are mounted on opposite faces of the insulating board 1c, as shown in FIG.4. Accordingly, the intensity levels of signals detected at the X axis sense-line electrode 1a and the Y axis sense-line 1b electrodes are different because of the difference between the distance la, lb from the signal source 6a of the coordinate indicator b to each of the sense-line electrodes 1a and 1b. It is known that the level of this signal is in inverse proportion to the square of the distance. therefore, as shown in FIG.4, when the X axis sense-line electrodes 1a is mounted on the top side of the insulating board 1c on which the signal source 6a is disposed, the signal intensity detected at the X axis sense-line electrode is different from the signal intensity detected at the Y axis sense-line electrode when it is input into the A/D converter 8 through the analogue circuit 7, and even when the A/D converter 8 is sufficiently reached by the detected signal from X axis electrode sense-line electrode 1a sufficiently reaches the threshold level of the A/D converter 8, the detected signal from Y axis sense-line 1b will be small and insufficient to reach the threshold level, therefore the analysis capability of the Y axis coordinate reading device is reduced.
Also, when trying to obtain an input level of the detecting signal produced by the Y axis sense-line electrode 1b sufficient for the A/D cnverter 8, the detecting signal produced by the X axis sense-line electrode 1a becomes too large for the A/D converter 8.
In the prior art, to obtain a sufficient analysis capability of the X axis and the Y axis, a very thin insulating board 1c ( t=0.4-0.5 mm ) must be used to shorten the distance difference or gap .vertline.la-lb.vertline. between the respective sense-line electrodes 1a, 1b, and therefore the prior art device must be treated with much care due to its thinners, and cannot have high yield in manufacturing.
Meanwhile, to shorten the intensity level difference between the signals detected by the respective sense-line electrodes 1a and 1b, the distance between the signal source 6a and the X axis sense-line electrodes 1a can be enlarged. Therefore, the ratio K of the signal levels is, when t=lb-la, ##EQU1## It is easily understood that when la is large as compared to t, the value K in the above formula becomes close to 1. However, keeping the signal source 6a away from the sense-line electrodes 1a and 1b will cause a decrease in the S/N ratio, and naturally cause lowering of the analysis capability. To maintain the analysis ability, the signal generated from the signal source 6a must be enlarged, and the analogue circuit 7 and A/D converter 8 shown in FIG. 3 must be made precisely, and consequently, the device becomes large in size and is very expensive.
To prevent these drawbacks, the present invention has the object to obtain sufficient analysis capability in both the X and Y axes, even when this coordinates reading device has a normal thickness insulating board (t=1.6 mm) disposed between the opposed sense-line electrodes.